Suicide bombers who carried out deadly attacks on three churches in Indonesia's second-largest city on Sunday were a family who had been in Syria and included two young children, police said, as the world's most populous Muslim nation recoiled in horror at one of the worst attacks on its Christian minority.

At least 13 people died in the attacks in Surabaya and at least 41 were injured, according to police, in acts that Indonesia's president condemned as "barbaric" and cowardly. It wasn't immediately clear whether the death toll included the family of six bombers.

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National Police Chief Tito Karnavian said that the father exploded a car bomb, two sons, ages 18 and 16, used a motorcycle for their attack, and the mother was with daughters, ages 12 and 9, for her attack.

Shattered glass and chunks of concrete littered the entrance of the church, which was sealed off by heavily armed police. Rescue personnel were treating victims on a nearby field while officers were inspecting wrecked motorcycles in the parking lot that were burned in the explosion.

The blast was followed by a second explosion minutes later at the Christian Church of Diponegoro that killed two people. Another two died in a third attack at the city's Pantekosta Church, Mangera said.

The bombings were the worst since a series of attacks on churches on Christmas Eve in 2000 killed 15 people and wounded nearly 100. Religious minorities, especially Christians, have been repeatedly targeted by militants.

The latest attacks in predominantly Muslim Indonesia came days after police ended a riot and hostage-taking at a detention center near Jakarta that left six officers and three inmates dead. The Islamic State group has claimed responsibility.

Indonesia has carried out a sustained crackdown on militants since bombings by al-Qaida-affiliated radicals in Bali in 2002 killed 202 people. In recent years, the country has faced a new threat as the rise of the Islamic State group in the Middle East invigorated local militant networks.